Hambergite

hambergite-mineral

Crystal system Orthorhombic
Transparency Transparent
Luster Vitreous
Fracture Uneven
Cleavage Perfect, Good Pinacoidal
Specific Gravity 2.35
Hardness 7.5
Optical Character Biaxial + ; Double Refractive
Refractive index 1.555-1.626
Birefringence 0.071
Dispersion 0.017
Fluorescence
Pleochroism
Chemical Formula Be2(OH)BO3
Comments
Streak

Another very rare beryllium mineral is called hambergite (HAM-berg-ite), named after Axel Bamberg. Its only source was thought to be the original locality in southern Norway; later, however, it was discovered in Madagascar and was first considered as a gem mineral when large colorless crystals were found there. Non gem crystals were discovered in the 1950s in pegmatite at Ramona, California. It has the necessary hardness to be an excellent gemstone (7 1/2), but it suffers from a lack of color and the fact that it has one direction of excellent cleavage and is so rare.

Hambergite is a beryllium borate, having the formula Be2(OH)BO3. It crystallizes in the orthorhombic system in prismatic crystals. The optic character is biaxial positive, with refractive indices ranging from 1.553 to 1.559 for the lowest index to 1.625 to 1.631 for the highest. The birefringence remains fairly constant at .072, which is exceptionally large for this point on the refractometer scale. Hambergite has low dispersion (.017) and a vitreous luster. It is remarkable not only for its high birefringence but for its low specific gravity (2.35). This great amount of doubling is sufficient to distinguish it from almost any other gemstone. Until such time as it is found in an attractively colored variety, hambergite will remain merely a curiosity among gem materials.

Transparent material is sometimes faceted, but the easy cleavage usually causes difficulty, causing the stone to crumble; for the reason, square cuts are not recommended. A tin lap with Linde A or a lucite lap with tin oxide will produce a satisfactory polish. 42° crown angles and 43° pavilion angles are suggested.